The invention relates generally to steam turbines and more specifically to maintenance operations requiring access to components within the exhaust hood of the steam turbine.
The outer shell of a steam turbine low-pressure section is generally called the exhaust hood. The primary function of an exhaust hood is to divert the steam from the last stage bucket of an inner shell to the condenser with minimal pressure loss. Usually the lower half of the exhaust hood supports an inner casing of the steam turbine and also acts as a supporting structure for the rotor. The upper exhaust hood is usually a cover to guide the steam to the lower half of the hood. The hood for large double-flow low-pressure steam turbines is of substantial dimensions and weight and usually is assembled only in the field. In many steam turbines, the inner case of the steam turbine, for example a double flow/down exhaust unit has an encompassing exhaust hood split vertically and extending along opposite sides and ends of the turbine. This large, box-like structure houses the entire low-pressure section of the turbine. The exhaust steam outlet from the turbine is generally conically-shaped and the steam exhaust is redirected from a generally axial extending flow direction to a flow direction 90 degrees relative to the axial flow direction. This 90-degree flow direction may be in any plane, downwardly, upwardly or transversely. Thus the exhaust hoods for steam turbines constitute a large rectilinear structure at the exit end of the conical section for turning and diffusing the steam flow at right angles.
The lower half of the exhaust hood, split horizontally from the upper half, directs the exhaust flow of steam to a condenser usually located generally beneath the exhaust hood. The lower exhaust hood may support the inner casing of the turbine and the associated steam path parts such as diaphragms and the like. The lower exhaust hood is further loaded by an external pressure gradient between atmospheric pressure on the outside and near-vacuum conditions internally. The lower exhaust hood shell is generally of fabricated construction with carbon-steel plates. Typical sidewalls for the lower exhaust hood are flat and vertically oriented. To provide resistance to the inward deflection of the sidewalls under vacuum loading, the lower exhaust hood traditionally has included internal transverse and longitudinal plates and struts. These internal transverse and longitudinal plates and struts form a web, generally underneath the turbine casing and extending to the sidewalls.
FIG. 1 illustrates typical arrangements of a prior art low-pressure double-flow steam turbine 5 with an exhaust hood 10. The exhaust hood 10 includes an upper exhaust hood 15 and a lower exhaust hood 20, mating at a horizontal joint 22. A turbine inner casing 25 may be supported on the lower exhaust hood 20. Various supporting structures are present in the form of transverse plates 40. These transverse plates 40 avoid the suction effect of the sidewalls 45 and end walls 50 and they distribute the load applied on the hood due to loads on inner casing 25. The lower exhaust hood 20 further provides a support location for shaft seals (not shown) and end bearings 75 for the turbine rotor 70. The lower exhaust hood 20 may include a framework that rests on the external foundation (not shown). Bearing housings 75 for the turbine rotor 70 are provided at axial ends of the exhaust hood 10.
A steam inlet 30 may penetrate a top of the upper exhaust hood 15 and include a seal 55 with the upper exhaust hood. The steam inlet 30 admits steam into steam chest 35 of the turbine inner casing 25. The steam inlet 30 is usually fabricated integral to the inner turbine casing 25. However, a removable steam inlet assembly 130 (FIG. 9) may be provided that includes a flanged joint 131 (FIG. 9) at the steam inlet to the turbine inner casing. This removable steam inlet assembly 130 may be detached from the steam inlet and attached to the upper exhaust hood with a lifting fixture as described in Docket 245789 by Chevrette and assigned to General Electric Company. Steam from steam inlet 30 is directed by series of fixed stator vanes 60 to rotating blades 61 for driving a turbine rotor 70. Steam exhausts from the turbine inner casing 25 at last stage blades 65.
In the constructing of an effective exhaust hood for use with such an axial flow turbine it is desirable to avoid acceleration losses within any guide means employed therein and to achieve a substantially uniform flow distribution at the discharge opening of the exhaust hood for the most efficient conversion of energy in the turbine and effective supplying of exhaust steam to the condenser to which it is connected. The static pressure at the discharge side of the diffuser will be higher than that of the exhaust hood discharge by the amount of pressure drop required to turn the flow from nearly axial to vertical and by the necessary pressure drop caused by passage of pipes, struts, and other such interferences.
A generally bell-shaped steam guide 90 may direct exhaust steam from the outlet of the turbine inner casing 25. The lower half of the steam guide 91 directs the exhaust steam downward into the lower exhaust hood 20. An upper section of the steam guide 92 exhausts upward to the top of the upper exhaust hood 15. At the top, much of the flow must be turned 180 degrees to place it over the steam guide 90 and inner casing 25, then turned downward. Pressure at the top is thus higher than at the sides, which are in turn higher than at the bottom. Structures within the upper exhaust hood may facilitate and smooth the turning of the exhaust steam downward to the exhaust hood outlet to a condenser below (not shown).
FIG. 2 illustrates an isometric cutaway view of a prior art upper exhaust hood 100. This particular upper exhaust hood includes a frame 110 supporting shell 150 enclosing the top of the upper exhaust hood. Endwalls 124 enclose the ends of upper exhaust hood. Horizontal joint 122 includes mechanical closure elements 130 to join with lower exhaust hood (not shown). The upper exhaust hood 100 also includes a butterfly plate 182. The butterfly plate 182 may include a first plate portion and a second plate portion 186 coupled to first portion 184. In the exemplary illustration, plate portions 184 and 186 are mirror images of each other. In another variations, butterfly plate 182 may be of unitary construction. More specifically, in the exemplary illustration, butterfly plate 182 has a substantially elliptical cross-sectional profile. Inlet steam entering inlet 30 (FIG. 1) passes through center opening 140 of shell 150 and is directed by an inner cylinder/shell (FIG. 1) through the steampath of turbine inner casing below (not shown). When the steam exits the last stage of the turbine inner casing substantially axially, the steam contacts the shell endwall 124 and reverses direction. Butterfly plate 182 directs the steam in the upper exhaust hood 100 into the lower exhaust hood (FIG. 1) and subsequently into the condenser. Additionally, butterfly plate 182 facilitates limiting an amount of exhaust steam, which is at a cooler operating temperature than the inlet steam, from contacting hot steam inlet surfaces. Butterfly plate portions 184 and 186 each may extend radially inwardly from casing inner surface 172 to a contoured radially inner surface 182 of portions 184 and 186.
A pair of support structures 200 may extend radially inward from an inner surface of each butterfly plate portion 184 and 186. Support structures 200 may include a center support rib (not shown) that extends between each respective plate portion 184 and 186 to opening 140, and a pair of side supports (not shown) that extend between center support rib and hood inner surface 172. Accordingly, support structures 200 provide structural support to butterfly plate 182, such that the steam flow path external to plate portions 184 and 186 remains relatively unimpeded. For other structural arrangements of an upper exhaust hood, different structural connections may be provided to support the butterfly plate from the upper exhaust hood. For example but not shown, the butterfly plate may extend and be supported from a sidewall of the upper exhaust hood 100. In all such arrangements, when the upper exhaust hood 100 is lifted, the butterfly plate 1182 is also lifted since it is mechanically attached to the upper exhaust hood 100.
When access is required to the inside of the exhaust hood 100 or inside the turbine inner casing 25, access may be provided through man-way covers 230 or other provided man-way access points. For major work within the exhaust hood or removal of major components, the upper exhaust hood 100 may be removed. Such access may be required for preventive maintenance, repair maintenance or modification. Due to the significant size and weight of the upper exhaust hood, means for lifting, such as a heavy-duty overhead crane, is often used to perform the lifting. Studies performed to analyze construction cost of a gas turbine power plant suggests that about $300,000 to $350,000 per meter of facility height or up to about $10,000 per inch of facility height is required to provide concrete block walls for such a facility.
Accordingly, it would be desirable to provide turbine equipment and methods for limiting required lift height and thus allow lower power plant wall height and hence lower facility costs.